Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $250.97   8.189%   $271.5219   $271.52   $271.50   $271.50  
  Purchase #2 $109.51   9.459%   $119.8686   $119.87   $119.85   $119.90  
  Purchase #3 $240.50   13.573%   $273.1431   $273.14   $273.15   $273.10  
  Purchase #4 $12.99   9.844%   $14.2687   $14.27   $14.25   $14.30  
  Purchase #5 $58.53   3.534%   $60.5985   $60.60   $60.60   $60.60  
  Purchase #6 $27.48   8.313%   $29.7644   $29.76   $29.75   $29.80  
  Purchase #7 $167.89   9.591%   $183.9923   $183.99   $184.00   $184.00  
  7 purchase totals = $867.87       $953.1575   $953.15   $953.10   $953.20